Biomedical Engineering Reference
In-Depth Information
Fig. 5.9
Illustration
of
the
enclosed-filling-problem:
a
force
response
of
an
under-filled
(non-intact) system, and b force response of an intact system (Benderoth 1984)
(homeostasis) based on the interaction of osmotically active proteins in the blood,
the hydrostatic pressure due to the pumping capacity of the heart and the expulsion
of water by the kidneys.
Numerous ex vivo and in vitro measurements have been conducted by (Fung
1993) to evaluate the mechanical properties of human tissue. However, extracting
tissue samples from their original surroundings the (mechanical) properties change
dramatically due to change of pretension, pH-value and/or concentration of
electrolytes and water (Fung 1993).
Figure
5.9
qualitatively illustrates the different force-displacement relations
obtained from a living (intact) and an under-filled (not intact) organism. The not
intact organism is characterized by a decidedly slower increase in the origin
(horizontal tangent). An adequate description of the mechanical properties of
human tissue and their mechanical characterization for a continuum mechanical
material model requires measurements involving the living organism in vivo. This
is indispensable.
Figure
5.10
a and b depict characteristic stress-strain data based on ex vivo
samples of elastin, collagen and passive and active muscle, as well as stress-stretch
data of the skin. The curves show that such materials generally exhibit strongly
non-linear material behaviour with an initial low load increase and an approxi-
mately linear initial region and an exponential load increase at higher strain.
In addition, the qualitative curve characteristics correspond to the characteristics
expected from a not-intact organism (Fig.
5.9
a).
